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Problems caused by intermittent earth fault are normally seen in compensated network substations: an
earth fault can trip multiple feeders simultaneously, or an entire substation can be tripped by residual
voltage back-up protection from the incomer. This is typical of old-fashioned relay protection as it is not
capable of differentiating between a normal consistent earth fault and an intermittent earth fault. As the
intermittent earth fault is a transient type of fault where the actual fault lasts only for a few hundred
microseconds, this causes traditional directional earth fault protection relays to lose their directional
sensitivity, and as a result their directional decision algorithms go haywire and the trip decisions will be
completely random. Typically, when a whole substation goes dark the logs of all protection relays show
how they have experienced multiple incorrect directional earth fault starts and releases, as well as an
incomer relay residual voltage trip. This is also the worst case scenario. In another typical scenario a
few feeders, including the correct faulty feeder, have tripped at the same time. In this case, as in the
previous, all the relays' logs show various incorrect directional earth fault starts and releases.
Previously, these scenarios were usually ignored and filed under 'Mysteries of the universe'
because they only occured once or twice a year and because disturbance recordings were not
commonly used in normal medium-voltage substations for fault verification. However, when disturbance
recorders were introduced as a common feature of protection relays this phenomenon received a name
and defined characteristics. One such characteristic is the occurence of high magnitude current
spikes, which –compared to residual voltage– are in the opposite direction of the current spike in
faulty feeders and concurrent in healthy feeders. Handling these unique characteristics requires a
completely different set of tools than what traditional directional earth fault protection can offer. The
following figures present three intermittent earth fault situations experienced by relays in a substation..
Figure. 5.3.7 - 52. An intermittent earth fault in a medium size network tuned close to resonance, as seen by a faulty feeder relay.
A
AQ
Q-C255
-C255
Instruction manual
Version: 2.06
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© Arcteq Relays Ltd
IM00035
Содержание AQ-C255
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